The term “nanotechnology” generally refers to objects, systems, mechanisms and assemblies smaller than one ten of micron and larger than 1 nm. In recent years nanotechnology has been used to make products, that is, raw materials are processed and manipulated until the desired product is achieved. In contrast, nanotechnology mimics nature by building a product from the ground up using a basic building block—the atom. In nanotechnology atoms are arranged to create the material needed to create other products. Additionally, nanotechnology allows for making materials stronger and lighter such as carbon nanotube composite fibers.
One of the areas of continuous development and research is an area of energy conversion devices, such as for example secondary batteries capable of charging electricity after discharge and having at least one electrochemical cell. The cell includes a pair of electrodes and an electrolyte disposed between the electrodes. One of the electrodes is called a cathode wherein an active material is reduced during discharge. The other electrode is called an anode wherein another active material is oxidized during discharge. Secondary batteries refer to batteries capable of charging electricity after discharge. Recently, intensive research has been conducted on lithium secondary batteries because of their high voltage and high energy density. The typical lithium metallic or lithium ion battery has an anode containing an active material for releasing lithium ions during discharge. The active material may be metallic lithium and an intercalated material being capable of incorporating lithium between layers. The active material is deposited or coated upon a metal current collector formed from a metal tape to increase electro-conductive characteristics of at least one of the electrodes.
Alluding to the above, various methods for deposition of the active materials onto the metal current collector have been used in the prior art applications. One of these methods is physical vapor deposition (PVD), which includes E-beam evaporation, flash (thermal) evaporation, plasma evaporation and DC or RF sputtering deposition, is currently used to generate thin films on substrates, i.e. the metal current collector. However, this method includes numerous disadvantages, such as, for example, non-time effective deposition rates as relate to coating thickness of the substrate per unit, typically in the range of a few microns per minute. Another method is known as chemical vapor deposition (CVD), including rapid thermal CVD, or RT CVD, results non-time effective deposition of the coating onto the substrate. Sputtering techniques such as RF or DC sputtering, as well as laser evaporation, plasma arc evaporation, electro-spark deposition (ESD), and the like are also known to have low deposition rates or not sufficient adhesion strength to the current collector. Moreover, all of the aforementioned methods are performed by and require expensive vacuum equipment and do not provide strong adhesion of the coating to the substrate, which is detrimental in various applications, particularly in manufacturing electrodes for energy conversion devices, such as batteries.
These aforementioned methods are proven to achieve rates of tens of microns per minute. However, if the deposition rates of these methods are increased to higher rates, it may adversely impact adhesion of the coating upon the substrate. As such, these methods are limited to deposition of the coating that results in a range of 10-20 μm per minute, which has limited industrial application, such as to production of a very thin battery of the type used in electronic devices. However, these prior art methods are not cost effective when used in a production of other types of batteries, such as, for example, batteries for vehicles, and the like.
Alluding to the above, another method, which uses vacuum, was also applied in fabrication of the substances of the electrodes. However, this method had negatively impacted the crystalline composition of the materials deposited upon the substrate. Those skilled in the art will appreciate that a shortage of oxygen in spinel phases leads to transformation of cubic crystal matrix to tetragonal one, which negatively affects electrochemical properties. The usage of carbon as a conductive agent, in some of the prior art applications, presents numerous disadvantages because of the lower electrical conductivity of the carbon as compared to metals, thereby creating additional voltage drop at the interface with the metal current collector.
The art is replete with various other methods and apparatuses for forming metal current collector for electrodes of a battery cell, which are disclosed in the United States Patent Publication Application Nos. 20020177032 to Suenaga; 20030203282 to Grugeon; 20040248010 to Kato et al.; and the U.S. Pat. No. 6,761,744 to Tsukamato et al. Other United States Patent Publication Nos. 20040197654 to Barker et al.; 20040202937 to Barker et al.; 20040185343 to Wang et al.; and 20040137326 to Munshi disclose various methods of electrode production, which use coating process, wherein ground particles of active substance and current conducting material are mixed with solution and organic binder to form a paste-like mixture or slurry. A current conducting collector is covered with the paste-like mixture, rolled, and dried. The electrodes used by the aforementioned “coating” method are known to have limited points of contact as defined between the ground particles of active substance and current conducting material and the current collector which drastically reduces the porosity of the current collector. The U.S. Pat. No. 6,800,399 to Matsumoto teaches an electrode substrate formed by mechanically processing a nickel foil so as to be made three dimensional through the creation of concave and convex parts, and then, this substrate is filled with active material or the like so that an electrode is manufactured. The concave and convex parts are rolling pressed to incline in one direction. Furthermore, an electrode for a secondary battery is formed by using the above described method. This method has a major disadvantage such as low permittivity due to a large amount of metal constituent. This method is non-cost effective because it depends upon special mechanical processing and equipment.
These aforementioned prior art methods share at least one disadvantage such as the active layer formed on top of the metal current collector of the electrodes to define a space therebetween, which negatively impacts specific power and energy, cycleability and possibility to properly function in applications requiring higher C-rate. The aforementioned methods negatively impact both the life span of the battery and the manufacturing costs associates therewith is the structure of the battery wherein the active layer is formed on the metal current collector and additional binders used as adhesion between the active layer and the metal current collector thereby increasing both the weight and size of the battery, which, as mentioned above, negatively impacts both the impedance characteristics of the battery and the manufacturing costs associated therewith.
Alluding to the above, none of these prior art references teaches the method of forming the electrode which would satisfies majority of the requirements, such as, for example, accessible porosity sufficient for penetration of electrolyte to contact with particles of the active material, a conducting agent, which should provide contact of the active material with a metal current collector and constitute no more than 10-20% of the active substance weight. In the normal process of gas dynamic evaporation accelerated to supersonic speed metallic particles form a coating on the substrate as a result of their plastic deformation. The usage of this method for electrodes production is significantly limited. The ceramic particles of active material are inculcated in the metal current collector and do not form the necessary porosity. Mixing a metal powder the ceramic components results in plastic deformation of the metal particles as they collide with the ceramic particles. In response to the plastic deformation, the metal particles create films on the ceramic particles resulting in low adhesion strength. The resulting material does not have sufficiently accessible pore structure and have poor mechanic strength.
But even with the aforementioned technique, to the extent it is effective in some respect, there is always a need for an improved processes for engineering of porous electrodes that is light, thin, cost effective, have improved life-span and ability to properly function in applications that depend upon higher C-rate and easy to manufacture.